Operator control over heavy equipment such as cranes used in the construction of high rise buildings, presents many problems. The simple movement of a lever by an operator is required to maneuver many tons of equipment delicately to a precise location. The movement of the equipment is accomplished through flow of high pressure hydraulic fluids through hydraulic lines that force movement of hydraulic motors that in turn force rotation at various pivotal joints provided in the equipment.
It is not always feasible to provide direct manual control over the hydraulic valve and a form of pilot control over the hydraulic valve remote from the hydraulic valve is common. The particular type of pilot control contemplated herein is an air control valve (the controller valve) regulating the air pressure applied to a double acting, self centering cylinder (the actuator).
Typical for both the controller and the actuator is the use of control springs. The control spring in the controller valve establishes the PSI in the conduit and responds to the lever setting. The control spring in the actuator establishes the degree of movement of the spool in response to air pressure in the connecting conduit and thereby the extent to which the hydraulic valve is opened. Thus, an operator starts out with the lever of the air control valve in the closed position and accordingly the hydraulic valve is also in the closed position. Movement of the lever effects depression of the controller's control spring which determines the air pressure in the conduit. The control spring in the actuator is depressed by the conduit air pressure and that depression determines the extent of opening of the hydraulic valve.
The problem with the coupled controller valve and actuator cylinder is the development of operator feel and the match up between the two units. It is desirable for the operator to be able to vary the lever position of the controller between fully closed to fully opened positions and get a similarly varied responsive movement for the hydraulic motors that are operating the equipment. If the equipment needs to be adjusted slightly, a slight movement of the lever should produce slow deliberate movement of the hydraulic motor. Similarly, a full movement of the lever should produce maximum rate of movement of the hydraulic motors with in between positioning of the lever accomplishing a proportional rate of movement of the hydraulic motor.
It is known that all controllers (pressure regulating valves) operate near a threshold of instability. The instability is normally deadened due to a significant hysteresis (lagging response) which is inherent in most controllers. To enhance the performance of a controller, the hysteresis must be reduced to a minimum. In doing so the threshold of instability may not be adequately dampened. When the threshold is crossed, a loud annoying harmonic is initiated. The harmonic is usually initiated by a rapid lever movement to a mid-range setting. Once initiated, the harmonic is self propagating and will not end until the lever is returned to the center position. In some controllers of the past the harmonic would not cease until the supply air was terminated. In addition to the displeasure of the operator with the noise, the controlled equipment is not responsive and damage to the regulating section of the controller is likely to occur. When the harmonic is initiated, the output pressure is in effect dithered and the average pressure remains low. The combined mass of the actuator and the spool coupled with the biasing force of the control spring provides a resistance too large even for the momentary spiked pressure set up by the harmonic. Therefore the controlled equipment does not respond to lever displacement on the controller as would be expected by the operator. Obviously, such harmonics are to be avoided.
A second problem is that the hydraulic valve and actuating cylinder is typically developed by the hydraulic valve manufacturer and the air control valve by the control manufacturer. In one case the air valve may be set to be wide open and hydraulic valve only partially open thereby preventing maximum utilization of the hydraulic motor. In the alternate case, the hydraulic valve will be wide open and the air control valve only partially open thereby reducing the control capability for the operator, i.e. the effective stroke of the lever is shortened and operator "feel" is made more difficult.